How Plants Produce Dioxygen

Almost four billion years ago, some unicellular organisms developed the ability to use light from the sun as a plentiful energy source. By combining light energy and available sources of chemicals, these organisms evolved into the first photosynthetic species. Most people associate the term "photosynthesis" with the ability of green plants to use energy from light to power the manufacture of sugars and other organic molecules. However, what is sometimes forgotten is that photosynthesis also supplies all of the molecular oxygen, or dioxygen (O2), we breathe. For this reason alone, it is probably not surprising that understanding O2 production by plants and other photosynthetic organisms has long been a focus of research in photosynthesis. The surprising part is that we still do not know exactly how plants generate O2, even though its role in photosynthesis was discovered more than 200 years ago.

To be sure, much has been learned in that time. Scientists have found that chloroplasts are the subcellular organelle in which photosynthesis takes place. They have even discovered many of the proteins and inorganic molecules that carry out the energy conversion. And yet, when it comes to the very last step, the one where dioxygen is actually formed, scientists are still working out the precise molecular mechanisms. At the core of their studies is a manganese-containing complex embedded in the chloroplast membranes, called the oxygen-evolving complex. The structure of this complex is still largely unknown, but some of its features can be deduced and modeled. These deductions have led, in turn, to some interesting proposals about the way plants produce oxygen.